Corn (Zea mays L.) is the most important and abundant crop produced in the United States. Corn is used as human food, livestock feed, and as raw material in industry. The food uses of corn include kernels for human consumption, dry milling products such as grits, meal and flour, and wet milling products such as corn starch, corn syrups, and dextrose. Corn oil recovered from corn germ is a by-product of both dry and wet milling industries. Both grain and non-grain portions of corn plants are used extensively as livestock feed, primarily for beef cattle, dairy cattle, hogs, and poultry.
Corn is used to produce ethanol while corn starch and flour are used in the paper and textile industries. Corn is also used in adhesives, building materials, foundry binders, laundry starches, explosives, oil-well muds, and other mining applications. Plant parts other than the grain of corn are also used in industry; for example, stalks and husks are made into paper and wallboard and cobs are used for fuel and to make charcoal.
The goal of a corn breeder is to improve a corn plant's performance and therefore, its economic value by combining various desirable traits into a single plant. Improved performance is manifested in many ways. Higher yields of corn plants contribute to a more abundant food supply, a more profitable agriculture and a lower cost of food products for the consumer. Improved quality makes corn kernels more nutritious. Improved plant health increases the yield and quality of the plant and reduces the need for application of protective chemicals. Adapting corn plants to a wider range of production areas achieves improved yield and vegetative growth. Improved plant uniformity enhances the farmer's ability to mechanically harvest corn.
Corn is a monoecious plant, i.e., corn has imperfect flowers—male, pollen-producing flowers and separate female, pollen-receiving flowers on the same plant. The male flowers are located at the top of the plant in the tassel, and the female flowers are located about midway up the stalk in the ear shoot. Each male flower has three anthers and each female flower includes a husk that envelops the cob and silks that emerge from the end of the cob and husks. Pollination is consummated by transfer of pollen from the tassels of the male flower to the silks of the female flowers.
Because corn has separate male and female flowers, corn breeding techniques take advantage of the plant's ability to be bred by both self-pollination and cross-pollination. Self-pollination occurs when pollen from the male flower is transferred to a female flower on the same plant. Cross-pollination occurs when pollen from the male flower is transferred to a female flower on a different plant.
A plant is sib-pollinated (a type of cross-pollination) when individuals within the same family or line are used for pollination (i.e. pollen from a family member plant is transferred to the silks of another family member plant). Self-pollination and sib-pollination techniques are traditional forms of inbreeding used to develop new inbred corn lines but other techniques exist to accomplish inbreeding. New inbred corn lines are developed by inbreeding heterozygous plants and practicing selection for superior plants for several generations until substantially homozygous plants are obtained. During the inbreeding process with corn, the vigor of the lines decreases and after a sufficient amount of inbreeding, additional inbreeding merely serves to increase seed of the developed inbred. Inbred corn lines are typically developed for use in the production of hybrid corn lines.
Natural, or open pollination, occurs in corn when wind blows pollen from the tassels to the silks that protrude from the tops of the ear shoot and may include both self- and cross-pollination. Vigor is restored when two different inbred lines are cross-pollinated to produce the first generation (F1) progeny. A cross between two defined homozygous inbred corn plants always produces a uniform population of heterozygous hybrid corn plants and such hybrid corn plants are capable of being generated indefinitely from the corresponding inbred seed supply.
When two different, unrelated inbred corn parent plants are crossed to produce an F1 hybrid, one inbred parent is designated as the male, or pollen parent, and the other inbred parent is designated as the female, or seed parent. Because corn plants are monoecious, hybrid seed production requires elimination or inactivation of pollen produced by the female parent to render the female parent plant male sterile. This serves to prevent the inbred corn plant designated as the female from self-pollinating. Different options exist for controlling male fertility in corn plants such as manual or mechanical emasculation (or detasseling), genetic male sterility, and application of gametocides. Incomplete removal or inactivation of the pollen in the female parent plant provides the potential for inbreeding which results in the unwanted production of self-pollinated or sib-pollinated seed. Typically, this seed is unintentionally harvested and packaged with hybrid seed.
The development of new inbred corn plants and hybrid corn plants is a slow, costly interrelated process that requires the expertise of breeders and many other specialists. The development of new hybrid corn varieties in a corn plant breeding program involves numerous steps, including: (1) selection of parent corn plants (germplasm) for initial breeding crosses; (2) inbreeding of the selected plants from the breeding crosses for several generations to produce a series of inbred lines, which individually breed true and are highly uniform; and, (3) crossing a selected inbred line with an unrelated line to produce the F1 hybrid progeny having restored vigor.
Inbred corn plants and other sources of corn germplasm are the foundation material for all corn breeding programs. Despite the existence and availability of numerous inbred corn lines and other source germplasm, a continuing need still exists for the development of improved germplasm because existing inbred parent corn lines lose their commercial competitiveness over time.
Traditional plant breeding to include parental lines for desirable traits involves crossing selected parental lines to introduce those desirable traits into the progeny of the cross. In a crossing-based approach, often, not only the desirable trait is transferred to the progeny but some randomization of the genomes of both parental lines occurs. This results in a wide segregation and variation of morphology and other traits of the progeny, which are not predictable. The uncontrolled variation renders the progeny selection process very long, cumbersome and laborious especially if the desired traits are not expressed early in the progeny or if the desired trait is recessive.
In an effort to minimize random variation, breeders prefer homozygous parental lines (inbreds) so that the genetic makeup of the F1 generation is more predictable. The inbreds with a desirable trait are generated by back-crossing a heterozygote with its parental lines, followed by segregation selection and repeated back-crossing. However, this repeated back-crossing is also very long, usually up to 6-7 times, depending on the plant, would produce a homozygous plant with the desired trait. Of course, the time scale involved here is dictated by the rate at which plants grow to maturity and set seed and several years can be necessary to produce the desired homozygous parent line.
Haploid plants contain one half of the usual complement of genes. Normal plants are diploid in that they have two complete sets of chromosomes, one from each parent. Haploid plants are capable of growing to maturity but are generally sterile. There are several known methods of generating haploid plants. One method is to generate maternal haploid plants by means of crossing a female parent with a haploid-inducer male parent, which results in a portion of the fertilized embryos being haploid for maternal chromosomes. These haploid embryos or subsequent plants can be selected using phenotypic markers. Thereafter, homozygous diploid plants are produced by the doubling of a set of chromosomes (1N) from the haploid tissue by exposure to a doubling agent, such as colchicine, nitrous oxide gas, heat treatment, and trifluralin. See, e.g., Wan et al., “Efficient Production of Doubled Haploid Plants Through Colchicine Treatment of Anther-Derived Maize Callus”, Theoretical and Applied Genetics, 77:889-892, 1989 and U.S. Patent Application No. 20030005479 the disclosure of which is expressly incorporated herein by reference. The doubling of chromosomes produces completely homozygous diploid plants, called doubled haploids. This method of doubled-haploid plant breeding eliminates the need for multigeneration inbreeding to produce a segregating population of inbred lines for evaluation, saving years of time.
By producing doubled-haploid progeny, the number of possible gene combinations for inherited traits is more manageable. Thus, an efficient doubled haploid technology can significantly reduce the time and the cost of inbred and cultivar development. The present invention addresses this need by providing a novel inducer corn line designated AX6012 that contributes to the production of haploid plants that can be subsequently doubled to yield highly uniform inbred lines.